Computer won't go into sleep mode

Dear sirs
I wonder if you can help me whenever I put my computer into sleep mode it shuts down and thenafter a couple of seconds it starts up again. I have tried various settings in the power options of the Control Panel but nothing seems to workk.
Could you please advise me in any way.
Yours Truly
P H Millward (goldenorbs)

Hi thanksto all who have responded to this query.
I am using Vista OS.
I have tried disconnecting the keyboard and the mouse immediately after pressing the sleep button. The computer goes into Sleep mode but comes back on straight away. I have also disconnected all other USB leads that are not in use and put it to sleep with the same result.

Do you have a screen saver ? Make sure that things like the screen saver is not set to come on at the same time as the the screen is turned off or other things that may be set to turn on/off at the same time.

Also - Check the peripherals in Device Manager (Properties - Advanced Tab) and make sure that the power option "Allow the computer to turn off this device to save power" is enabled.

Next, review the running processes to see if you can identify something that would be waking up periodically. You could also try disabling the wireless connection when you put it to sleep to see if the network activity is waking it up.